It is becoming ever more usual to use electronic controls in place of electromechanical controls which have been previously used to provide similar functions. Perhaps this change is most common in space heating and air conditioning control, where the standard thermostat using a bimetal temperature sensor to operate a mercury switch is more and more often replaced by an electronic thermostat. There are a number of reasons for replacing the electromechanical controls with electronic circuitry. The electronics may be more reliable, and certainly will provide more functions than an electromechanical control. The electronic control is usually much more accurate than the electromechanical control.
The invention addresses a particular problem that arises most often when replacing an electromechanical thermostat with an electronic unit in an existing structure, but may arise in other contexts as well. This problem is that there are frequently only two wires present in the existing structure for connecting the thermostat or other control to the controlled device and thus no convenient way to provide power for the electronic control. Providing power to operate the electronic control circuits at the remote location thus becomes a non-trivial problem.
In these situations, a variety of systems are now in use. It is possible to run extra wires from the controlled system. If standard electrical power is close by, the power may be derived from it. Some thermostats use so-called power stealing systems whose rechargeable batteries are charged when the internal thermostat switch is open. Other thermostats use low power circuitry operated by disposable batteries which must be periodically replaced.
All of these systems have some disadvantages. The power stealing units suffer failures of the rechargeable batteries and require a charging circuit. The need to replace disposable batteries is a nuisance. Running extra wires for thermostats is usually easy and cheap in new construction, but expensive and time-consuming in existing structures. And if the thermostat or other control device is located at an extremely remote location, then running more than two wires may still be disadvantageous. For all these reasons, it has been somewhat of a problem to provide electronic control and sensor circuitry at these remote locations, and in particular, in the case of sophisticated thermostat control in older structures. Ironically enough, it is these older buildings which are the ones which might most benefit from the energy conserving ability provided by the sophisticated temperature control provided by state of the art electronic thermostats.